1. Field of the Invention
The present invention relates in general to an ocular lens such as a contact lens placed on an eyeball or an intraocular lens inserted within an eye. More particularly, the present invention is concerned with a multifocal ocular lens having a plurality of vision correction regions having respective different values of optical or vision correction power.
2. Discussion of the Related Art
As an ocular lens used for vision correction of an eye suffering from deteriorated accommodation such as presbyopia, there has been proposed a multifocal ocular lens having a plurality of vision correction regions which have respective different values of optical power. As one example of the multifocal ocular lens, JP-A-63-95415 and JP-A-1-319729 disclose an alternating or translating vision type contact lens, wherein the vision correction regions with different values of optical power are selectively and alternatively used as need in observing objects, with a shift of the visual axis of the lens user or wearer. As another example of the multifocal ocular lens, U.S. Pat. No. 4,580,882 and JP-A-2-217818 disclose a simultaneous vision type contact lens, wherein the vision correction regions are simultaneously used, and one of the vision correction regions through which a desired image is observed is selected by an action of the lens wearer's brain.
In either of the alternating and simultaneous vision type ocular lenses described above, there is proposed: a so-called bifocal ocular lens having two vision correction regions, i.e., a near vision correction region through which near objects are observed, and a distant or far vision correction region through which distant objects are observed; and a multifocal ocular lens having such near and distant vision correction regions and at least one intermediate vision correction region which is interposed between those near and distant vision correction regions and which has an optical power value different from the optical power values of the near and distant vision correction regions. The multifocal ocular lens has at least three vision correction regions having respective different optical power values.
The bifocal ocular lens having the two vision correction regions, i.e., the near and distant vision correction regions, tends to suffer from a jump or overlapping of the image of an object which is located intermediate between the near and distant objects. This is because the bifocal ocular lens has only two focal points which correspond to the respective near and distant vision correction regions. Further, the bifocal ocular lens has surface discontinuity at a junction of the near and distant vision correction regions. In other words, the two vision correction regions are not smoothly connected to each other at the junction therebetween, with a result of deteriorating a wearing comfort as felt by the lens user.
In the multifocal ocular lens, each of the plurality of vision correction regions inevitably has a relatively small radial dimension, so that the optical power of the lens changes in steps in its radial direction, corresponding to the three or more vision correction regions. In this case, the near and distant vision correction regions do not have sufficiently large surface areas, whereby the images to be viewed through these near and distant vision correction regions undesirably tend to be blurred. Moreover, since the optical power in the at least one intermediate vision correction region changes in steps, the multifocal ocular lens does not assure a high degree of visual acuity of the object located intermediate between the near and distant objects, so that the multifocal ocular lens is likely to suffer from a problem of ghosting or double imaging.
JP-A-5-181096 discloses a multifocal ocular lens having a near vision correction region, an intermediate vision correction region, and a distant vision correction region, so that the optical power of the multifocal ocular lens continuously varies in its radial direction from the value of one of the near and distant vision correction regions to the value of the other of the near and distant vision correction regions. This multifocal ocular lens was developed in an attempt to improve its wearing comfort as felt by the lens user, and assure clear view of the objects located intermediate between the near and distant objects. However, the proposed multifocal ocular lens is still unsatisfactory in providing sufficiently clear viewing of both of the near and distant objects.
JP-A-9-26559 proposes a bifocal ocular lens having a plurality of pairs of near and distant vision correction regions which are arranged such that the near and distant vision correction regions are alternately located in the radial direction of the ocular lens. Like the ordinary bifocal lens having one pair of near and distant vision correction regions, the proposed bifocal ocular lens does not assure clear view of the objects located intermediate between the near and distant objects, and suffers from the jump or overlapping of the image.